Telephone-exchange system



Mar-. 13, 1923. 1,448,576

, VV. B. STRCKLER y TELEPHONE EXCHANGE SYSTEM Filed Mar. 19', 1921 I. l i, w

UNITED ST res! FFlQE.

WALTER B. STB-ICKLER, OF EAST ORANGE, 'NEW JERSEY, ASSGNOR TO WESTERN ELECTRIC COMPANY, INCORPGRATED, O NEW YORK, N. Y., A CORORATION F NEW YORK.

TELEPHONE-EXCHANGE SYSTEM.

Application filed March 19, 1921.

To all w 710m t may concern.'

Be it known that I, il/Yamini Srnionnnn,

a citizen of the United States, residing; at

East Orange, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Telephone-Ex change Systems, oit which the following is a full, clear, concise, and exact description.

This invention relates to telephone exchange systems, and more particularly to a cord circuit arrangement for establishing connections between telephone lines.

@ne of the objects of the invention is to provide an improved cord circuit arrangement for establishing connections between extension lines oit a private branch exchange7 and between such extension lines and trunk lines leading to a inain central oiiice, which central oliice may be either ot the manual or machine switching type.

Another object of the invention is to provide a cord circuit of the character just described, which is so arranged that in the establishment of incoming connections between trunk lines extending from the main central oiiice and extension lines, nonthrough supervision will be provided, after the connection. to the extension line is established, or in other words the subscriber upon the extension line will have cont-rol or". the Supervisory apparatus at the private branch exchange only, and the operator will have control of the supervisory apparatus at the main central oilice.

Another' object of the invention is to provide cord circuit of the character abovev described, in which where a subscriber at the private branch exchange hangs up the receiver after being` connected to a trunk line leading to the central otiice, the trunk line will be 'freed automatically7 for an incoming; call from the main central oiiice. and the cord circuit will be opened to prevent a ringing signal incomingr trom the main centralv oiice over the trunlr line tromvringingg' the bell of the subscriber at the private branch exchano'e, in case the connection has not been tallren down.

Another object of the invention is to provide a cord circuit of the character above described, which is so arrangedvthat in the establishment or" outgoing connections between extension lines and trunk lines leading to the ina-in central otlice, through super- Serial No. 453,724.

vision will be provided after the connection to the trunk line is established, or in other words the subscriber upon the extension line will have control of supervisoryT apparatus at the main central oilice as well as the snpervisory apparatus at the private branch exchange.

y One of the features ot the invention consists in the provision of means whereby on all calls over the trunk lines through supervision will be provided except in those cases in which a ringing key is actuated at the operators position to call a private branch exchange station.

Another feature of the invention consists in the provision ot' means whereby a private branch exchange subscriber can transfer a call which is incoming over the trunlr'linek without actuating the supervisory apparatus at the main central office. I

These objects as wellas other objects ot the invention may be attained by means ot the arrangement set forth in the following description and illustrated in the accompanying.; drawing. the figure of which illustra-tes a circ-uit diagram of one embodiment of the invention.

Referring4 to the drawing, a cord circuit C ior establishing connections at a private branch exchange is shown terminatingu yin plugs l5 and 50. The plug l5 is adapted to co-opera-te with the jack 10y ot trunlr line T leading' to the. main central'othce, or with jacks, such 75, of extension lines, such as L2. leadingto individual stationsof the private branch exchan je. The plug 50 is also adapted to cooperate with aclrs.v such 5l, o textension lines, such. as L3, extendingM to individual stations ofthe private branch exchange. The. trunk line T may terminate at a main central office at which connections are established either manually or by means of machine switching; apparatus. Ringiivr keys FK and BK are associated with. the front and back cords terminatingn in the plugs l5 and 50, respectively, for ringing the stations connected to the private br ch exchange when local connections are established. A key LK is also provided for associating the operators set with the cord circuit. the lrey being; so arranged thatwhen operated the conduct-ors of the front and bach cords-r` instead of being` directlyconnected together, are interconnected through the operators set, the operato-rs set beingv provided with the usual transmitter M and the receiver R, and in addition, an impulse sender or dial S which is provided in order that the operator may establish connections by means of the machine switching apparatus at the main central office.

Assuming that a subscriber at the main central office desires to be connected toa` subscriber at one of the extensions, as, for example, the extension L3 oi the private branch exchange, ringing current will .be transmitted over the trunk line T from the main central oiiice, thereby energizing the left-hand winding of relay 11. Relay 11 upon being energized, closes a locking circuit through its right-hand winding, and

also closes a circuit for the trunk line signaling device 12, both circuits extending through the lower contact of the jack 10. The operator at the private branch exch-ange, upon observing the glowing of the lamp 12 inserts a plug 15 into the jack 10, thereby opening the lower contact of the jack 1 0 and extinguishing the lamp 12. A circuit is now extended from battery, through re sistance 13, sleeve contacts of the jack 10 and plug 15,l and' thence through the windtending over the inner lett-hand alternatel ing of rela 24 and non-inductive resistance 7 8 in paral el to ground. Normally, battery is supplied to the tip and ring conductors of the cord circuit C over the inner back contacts of relay 24 and the upper and lower windings of impedance coil 16. When relay 24 is energized, however, in response to the answering or' an incoming call over the trunk T, the battery supply connections are open, and the tip and ring conductors leadingto the plug 15 are connected together through the upper and lower windings of the impedancel coil-16 by a connection excontacts of relay 24 and outer lett-hand contacts of relay 47. A bridge is thus established across thecord circuit for tripping themachine yringing apparatusl at the main central office, in the manner well kno-wn in the art. Relay 24 also operates to shortcircuit at its outer right-hand contacts, the winding` of the supervisory relay 25, which is connected inthe ring conductor of the front cord.

The operator may now actuate the key LK in order to connect her set with the cord circuit. When the key LK is in normal position, the tip conductors of the front and back cords are connected by a path extending from the tip contact of the plug 15, over the upper normal contacts of key FK, upper normal contacts of key LK, conductor 92,

upper normal contacts of relay 28, upper normal contacts of key BK to the tip contact of the plug 50. In a similar manner, apath extends from the ring contact ot the plug 15, lower normal contacts ot key FK,

outer right hand contacts of relay 24, lowermost normal contacts of key LK, conductor 93, lower normal contacts of relay 28, through the winding oit supervisory relay 34 and'serially connected resistance's, 30 and 31, in parallel, lower normal contacts ot' key BK to the: ring contact of plug 50.' Then the key LK is actuated however, the circuit above described is changed, so that the tip conductors oft the front and back cords are connected over a path extending from conductor 92, through the uppermost alternate contacts of key LK, upper normal contacts oi" relay 58, contacts 90 of key LK, and thence over the path previously traced. The ring connections on the other hand, extend from conductor 93, through the lowermost alternate contacts of key LK, lower normal contacts o1c relay 58, contacts 94 of key LK, andthenceover the path previously traced. The operators talking and listening set comprising transmitter M and receiver B respectively, and the associated induction coil 98 are bridged across the circuit just described so that the operator Iis enabled. to converse with the calling subscriber or operator at the distant central oiiice to determine what extension line is desired.

. The operator upon obtaining this information and ascertaining that a connection to the extension L3 is desired, restores the listening key LK to normal and inserts the plug 50 in the jack 51, thereby closingv a circuit 'trom ground, over the sleeve contacts of the jack 51 andA plug 50, armaturev and back contact of relay 34, and to grounded battery, through the lamp 40, which is now caused to glow. The operator may then actuate the ringing key BK to transmit ringing current to the desired subscriber, which closes a circuit Vtor relay 48 as [follows: ground, lowermost contacts ot key BK, winding of relay 48, and inner right-hand alternate contacts oi' relay 24 to grounded battery. Relay 4 8 in operating locks up over a path extending `from grounded battery, inner right-hand alternate contacts of relay 24, winding and inner right-bruid contacts of relay 48 to ground. Belay 48 in attracting its lett-hand armature opens the locking eircuit tor relay 47, which relay is energized in a manner which will be hereafter described.

U pon the response of the subscriber at the station associated with extension L3, a supervisory circuit is completed 'from the subscribers station through the cord circuit at the private branch exchange and over the trunk line leading to the main central ofthce. This4 circuit may be traced as follows: Afrom grounded side ot the battery (not shown) at the main central oi'lice` over the tip vconcluetor of the trunk line T, tip contacts of the jack 10 and plug 15, upper normal contacts of key FK, uppermost normal contacts otV key LK, conductor 92, upper normal contacts of relay 28, upper normal'contacts of key BK, tip contacts of plug 50 and jack 51, through the substation loop, ring contacts of jack 5 1 and plug 50, resistances 31 and 30, and the winding of supervisory relay 34 in parallel, lower normal contacts of relay 28, conductor 93, lowermost vnormal contacts of key LK, outer right-hand armature and front 4contact of relay 24, lower normal contacts of key FK, contacts of plug 15 and jack 10, and thence over the ring conductor of the trunk line T to battery not shown) at the main central oliice. As is well known, the circuit over the trunk line T, if said line leads to a manual otlice, may include a supervisory relay in a cord circuit in the main central otlice. In case the main central oliice is of the machine switching type, a relay corresponding somewhat in function to the supervisory relay will be included in the machine switching circuit. The supervisory relay at the main central oilice will, on an ,incoming call, be under the control of the operator of the private branch exchange.

Thesupervisory relay 34 upon being energized opens the circuit of lamp 40 and closes a circuit through relay 47, which may be traced as follows: grounded battery, inner right-hand alternate contacts of relay 24, winding otrelay 47, front contact and armature ot relay 34, sleeve contacts of plug 50 and j ack 51 to ground. Relay 47, in attracting its outer right-hand armature prepares a. circuit for relay 28; in attracting its inner left-hand armature, short-circuits the noninductive resistance 30 shunted about -the supervisory relay 34; and in attracting its outer lett-hand armature substitutes for the low impedance bridge, through thev upper and lower windings oit the impedance coil 16, a high impedance bridge comprising the upper, lower and intermediate windings of impedance coil 16 in series, said bridge exe tending from the tip conductor o1 cord circuit C, through the upper and intermediate windings of coil 16, outer right-hand contacts ot relay 48, outer lett-hand alternate contacts ot' relay 47, lower winding of coil 16 to the ring conductor o'lcord circuit C. By the substitution of the high for the low impedance bridge, the shunting eifect of the battery supply from the main central otlice to the subscribers instrument on the extension L3 is lessened and a better trans mission circuit is provided.

When the subscriber at the station of the extension line L3 hangs up his receiver. the supervisory circuit previously traced lis opened, and consequently the supervisory relay 34 is deenergized. Relay 34 in retracting its armature opens the circuit for relay 47 which relay thereupon deenergizes.

and again closes at its outer left-hand contacts the bridge including the upper and lower'windings of coil 16 across the cord circuit. The relay 34 upon deenergization also completes a circuit from ground, through the sleeve contacts of jack 51 and plug 50, armatureand back contact of relay 34 and ythrough the supervisory lamp 40 to grounded battery causing said lamp to glow to inform the private branch exchange operator that the connection is to be taken down. Due to the presence of the bridge across the cord circuit non-through supervision is attained and the machine switching apparatus at the main central othce is prevented from releasing. It on tht*l other hand, the main central oflice at which the trunk line T terminates, is a manual otlice, the supervisory relay (not shown) in the cord circuit at the main central oliice will be prevented from releasing.

The operator at the private branch exchange noting thedisconnect signal 40 may remove plugs 15 and 2O from their respec-` tive acks thereby removing the bridge from the trunk circuit T and causing the release ot the switches at the main central oiiice or of the supervisory relay thereat, depending on whether the main central office is of the machine switching or manual type, in the manner well known in the art.

It the subscriber of extension line L3 should for any reason desire to transfer the incoming call to another station of the private branch exchange he may flash the operator by opening and closing his line circuit, causing thereby the intermittent energiza` tion of relay 34 and the liashing of lamp 40. The operator noting the flashing signal may respond and transfer the call. It will be noted that in transferring the call the low impedance bridge remains connected across the cord circuit so that the supervisory ap paratus at the main central oliice is not affected.

Should a subscriber on an extension line such as L3 desire to be connected to a subscriber at the main central oliice, the operation is as follows. When the subscriber removes his receiver rom the switchhook the line relay 52 is Aenergized to close the circuit of the line lamp 53. VThe operator in re-` battery supply may be traced from grounded battery, over the inner right-hand normal contacts of relay 24, lower winding of iinpedance coil 16, conductor 93, lower normal contact of relay 28, through the winding ot supervisory relay 34, and non-inductive resistances 30 and 31 in parallel, lower Vnormal contacts of key BK, ring contacts of plug 50 and jack 51, through the calling substation loop, tip contactsv of jack 51 and plug 50, upper normal contacts of key BK, upper normal contactsgof relay 28, conductor 92, upper'winding 'of impedance coil/.16, and vvinner left-handnormal contactof .relay 2Llfto ground. The supervisory relay 34 .isenere gized overathe circuit ,just traced and opens the `circuit ofi-'the supervisory lamp 40, which .would lotherwise be closed over the sleeve contactsfof plugO-andfjack 51.

Thebperator may. now aetuate .the listen ing ykey LK, V4thereby connecting `her telephone set-to Pthecord circuit in the manner already described, so that'she lisenabled to converse with fthe calling subscriber. Upon ascertaining that the subscriberl desires `a connection through the .main central office', the operator will insert the plug 15 into the jack V:10. kA circuit is no-w completed for relay 24: over the npath previously described, whichrelay in operating disco-nnectsthe batter-y supply through the impedancecoil 16, so" that fthe current for the 'calling subscribers station will lnow be supplied directly from the maincentral office. Relay 24 also short-circuits the winding ot the supervisory relay '25, and at `itsinner rightliand alternate contacts connects battery to one sideof the winding of relay. L17, thea-other side of which is connected to groundthrougb the contacts yof 4supervisory relay 311: and sleeve conta-cts of plug and vcontact '5.1. Relay 117 thereupon energizes. .i

Further vaction by the private 'branch eX- change operator willv "now depend upon Whether or not the main central office is a manual office oran automatic office. In case the main central office is a :manual yoffice the line relay (notshown') associated with the trunk line T at lthe v'main central office will ,be energized vas soon as the plug 15 is inserted into the jack lO'by means of the through supervisory circuit already traced, if the listening key LK is in normal kposition. `=Iftlie lkey LK ris operated however, as would .normally be the case under 'the conditions being described, a circuit for said line relay would be completed .as follows: yfrom grounded'barttery ,'(not shown), through said line relay `(not shown) over the ring .con ductor of the `trunk `line T, ring contacts of jack lOxand plug 15, lower normal contacts of key FK, front contact and outerrighthand armature .of relay Q11, contacts 94C ot key LK, right-hand norma-l contacts .of 're-' lay 60, impedance coil 59,-contacts 90 of key LK, r,upper normal contacts of key FK, tip

contacts of plug 15 andjack 10, and over the tip conductor of the trunk lline T to ground (not shown) atthe-niaincentral office. It will beinoted that'in addition to the circuit just described for the line rel-ay at the main' central office, a parallel circuit through the calling -subscribers station isl provided. The operatorI at the distant central office being ap-prised through the line.

lamp (not shown) Vthat a call has come in,

a-nswersthe call in the usual manner, andthe private Vbranch exchange operator through her telephone set which is connected to the cord C over the contacts of the -fkey LK .is able to converse with zthe loperator at the main central office and inform .the operator the :number desired .so vthat the yoperator at the main central office may complete the vconf nection. @he .private branch exchange operator after having passed the call to the main central ofliceoperator, will restore the listening fkey LK toinormal, so that the talking circuit will be the same as previously traced for a call incoming from the main central office.

In case the r,main central office is of the machine switching type the operator at vthe private branch exchange after yhaving inserted the plug 15 into the jack .10 may then act-nate the impulse sender S ya number' ot times corresponding to the number desired at .the main :central ofliceforthe purpose `of setting the machine switching apparatus thereat. Upon 'the initial :movement of the dial Sfoff-normal contacts 69 theretof are closed, -thereby 4completing an obvious circuit for sloweto-release relay 68, which in energizingucompletes an obvious circuit for relay 60. Relay in attracting its lefthand varmature completes an obvious circuit for lrelay F58 which relay i-n-operating opens the conductive connection which lhad previously existed between the front and back conductors of the cord through the operators set. Battery supply Vis now furnished to theextension L3 ithrou'gh'the windings of' relays61 over acpath which 'maybetraced'as follows: from grounded battery, through the lower winding ofrelay 61, lower right-hand winding fof repeating c0il99., lower alternate contacts yof relay 58, Jlowermost alternate contacts of `key LK, and thence over the circuit .previously traced through the super-v visory relay 34, through the calling substation loop and thence over a path previously traced lthrough the uppermost .alternate contacts 'ofvfkey LK, upper alterna-te contacts of relay '58, upper right-hand winding of repeating coi-l 99, and upper winding of `relay 61 to ground. Supervisory relay 34e is lheld energizedoverrthis circuit during the actuation of the dial and zrelay 61 is now renergized. Relay 61 at its armature and front contactcompletes a holding circuit for relay 58 to maintain the connection thus 4established, independently of the relay .'60, whose energization depends upon the dial being off-normal. f

llVhen :relay ,'60 was ienerriZed, upon the S, right-hand alternate contacts of relay 60, contacts 94 or key LK, outer righthand armature and front contact of relay 24, lowei` normal contacts ot' key FK, ring contacts ot' lug l5 and jaclc l0, to the ring conductor of the trunk line T. i The dial operates in the well-known manner to interrupt the dialing circuit just traced, and operates the niachine switching apparatus at the main central oi'lice.

This dialing circuit it will be noted, is independent oli the supervisory circuit controlled over the extension L3. lirowever, at the conclusion ot' the dialing operation, it the operator for any reason should tail to restore the listening key LK, a holding circuit tor the trunk line T will be provided through the impedance coil 59 in the operators set, as this coil will be bridged across the tip and ring conductors oit the trunk line T, This bridge is closed by the deenergization oit the relay 60, which takes place after the dial operation ceases. lilith the circuit in this condition the calling subscriber is able to converse with the called subscriber, even though the listening key LK has not been restored by reason ot the lfact that the right-haud winding of ind-uca tion coil 98 in the operators is bridged. across the tip and ring conductors ot' the cord circuit in parallel, with the inipedance coil 5S) as soon as the slow-to-relcase relay 68 is released. The repeating coil 99 serves to inductively couple the iront and back conductors through the operators set so that the subscriber can talk through the repeat ing coil. lt will be noted that the circuit of the right-hand winding ot the repeating coil 99 was held open during` the dialing operation by the relay 68 to preventthe noise resulting from the dialing impulses being transmitted to the calling subscriber. lit', as before stated, the operator fails to restore the key to normal after having` dialed, she is informed of her failure Ato 'do so by hearing a ringing signalbet'ore the called party answers or by hearing coirfersation a't'ter the called party responds whereupon she restores the key LK to normal position.

At the completion or conversation, the hanging up ot the calling subscribers receiver causes the release or' the supervisory relay 34. It the trunk line leads to an automatic exchange, the machine switching apparatus by which the connection was set up at the main central o'fiice, will be released in the manner well known in the art. It on the other hand, the main vcentral o'Hice at which the line T terminates, is

a manual oliice, the opening of the super` visory circuit above traced, results in the deenergizat-ion ot a supervisory relay (not shown) in the cord circuit at the main central oliice, so that the operator such or'iice will be informed that the connection 1s to be taken down. Relay 34 upon deenergization opens at its armature and front con tact the original energizing circuit for relay 47. Relay 47 remains energized however, over a locking circuit including contacts 96 of key LK, lett-hand contacts of relay 48, inner right-hand contacts and winding of relay 47', and inner right hand contacts of relay 24. Due to relay 47 locking (the bridge is prevented from being placed across the cord circuit when relay 34 releases. The extension subscriber thus controis the release of the connection. Relay 34 in deenergizing also completes a circuit for supervisory lamp 40 over the path previously traced, causing said lamp to glow to in'torm the private branch exchange operaltor that the connection is to be taken down.

Another circuit is completed at the same time for relay 28 over a path extending from grounded battery, winding of relay 28, outer right-hand contacts of relay 47, back contact and armature of relay 34, and sleeve contacts of plug 50 and jack 5l to ground. Relay 28 thereupon energizes, thereby disconnecting the front from the back cord and connecting ground and battery to the tip andv ring conductors respectively of the back cord. `By thus opening the cord circuit, ringing current transmitted over the trunk T from the main central ollice will not, in the extent that the private branch exchange operator delays taking down the connection, be transmitted to the extension L3 with which the cord circuit is still associated, to ring the bell at the subscribers station. The ringing response relay 26 is bridged across the front cord between the point of disconnection and the plug l5, so that this relay will respond to any ringing signal of this character, and will cause the lamp 71 to glow thereby giving the private branch exchange operator a recall signal.

Il. the subscriber at the extension L3 should recall by taking the receiver olf the hook before the connection has been taken down, a circuit is provided for energizing the supervisory relay 34 due to the operation of relay 28 so that by flashing the switchhook the lamp 40 may be caused to glow to attract the attention of the operator. The supervisory relay 34 when again energized by the subscriber removing his receiver before the private branch exchange operator takes down the connection, opens the circuit of relay 28. 'Relay 28 releases and again closes the cord circuit so that the connection extends directly to the main central otiice, and battery for supervisory relay 34 is now supplied therefrom. With the circuit in this condition the subscriber may now extend one or more calls through the distant central office without the aid or intervention of the private branch exchange operator.

It' no recall signal, as has been described, Should be transmitted, the private branch operato-r, upon observing the glowing of the lamp 40 in response to the hanging up of the receiver associated with the extension L3 will withdraw the plugs 15 and 50' from their respective jacks. By withdrawing the plug 15 the supervisory lamp 40 will be extinguished and the relay 28 will be deener- `gized, so that the talking conductors of the cord ycircuit will be restored to their normal condition. 4When the plug 15 isdisc-onnected, the relay Q4 is accordin; 2,lyv deenergized to again bridge battery across the conductors and to open the circuit of relay 47, there b-y restoring the cord circuit apparatus to normal.

In case .the subscriber o-f the extension L3 should desire to be connected with another extension L2 of the same private branch exchange, the operator will insert the plug 50 into the jack 51 and actuate the. listening key LK as before. Relay 24 of the cord circuit not being operated, battery is connected through the upper and lower windings of impedance coil 16 to supply current to the calling subscribers transmitter, and to operate the supervisory relay 34. The operator upon ascertaining the number of the described subscribers line will insert the plug 15 into the jack 75. The sleeve Contact Vof the jack 7 5 being grounded, the relay 24 is not energized over the sleeve circuit as in the case previously described, and conse quently battery is supplied for the called subscribed through thev impedance coil 16.

lRelay 47 is not energized by the supervisory relay 34during a connection of this character, for the reason that the circuit for relay 47 depends upon the energization of relay 24. The lamp 71 now functions as a supervisory lamp for the calling end of the oord circuit. A circuit is completed entending from battery` through the lamp, over the outer left-han d contacts of relay 24, back contact and armature of relay 25 and sleeve contacts of plug 50 and jack 51 to ground. When the called subscriber of the extension L2 responds, the supervisory relay 25, which is included in the battery supply circuit to the called subscriber. and which is, in this instance not short-circuited by the. righthand contacts of relay Q4, becomes energized and opens the circuit of supervisory lamp 71. When the two subscribers hang up, the supervisory relays 25 and 84 which are separately controlled by the two subscribers, become deenergized and the lamps 71 and 40 are again caused to glow to give the disconnect signal. end of the cord terminating in the plug 50 was usedasi an answering end, and the end of the cord terminating in the plug 15 as the calling end, the cord circuit will function equally well for the extension interconnec- IVhile as above described, the' tions if the plug 15 be used as the ans\ver-.

local connections it will be noted also that the feature of opening the cord circuit by means of the relay 28 inresponse to a disconnect signal is not present.

llhat is claimed is 1. In a telephone exchange system, a trunk line, a subscribers line, a cord circuit, means for providing` through-supervision over the cord circuit if calls are extended in one direction over said cord circuit, and means for .providing non-through-supervision if calls are extended in theother di rection over said cord circuit.

2. In a telephone exchange system, a trunk line. a subscribers line, a cord circuit, means for providing through-supervision over the cord circuit if calls'are extendedin one direction over said cord circuit, means for providing non-through-supervision if calls are extended in the other direction over said cord circuit, and means automatically operative incident to the establishment of a connection between said trunk line and said subscribers line via said cord circuit for converting said cord circuit from one type of supervision to the other.

3. In a telephone exchange system, a trunk line, a subscribers line, a cord circuit for interconnecting saidlines, normally arranged to provide through-supervision, and means responsive to an act necessary to extend an inward call over said trunk line to said subscribers line to convert the cord circuit to non-through-supervision.

4. In a telephone yexchange system, a trunk line, a subscribersline, a cord circuit for interconnecting said lines arranged to provide through or non-throughsupervision, and means operating automaticallyy to convert the cord circuit from one class of supervision to the other.

5. In a telephone exchange system, a trunk circuit, a line circuit, a station there on, a cord circuit connecting said trunk circuit and said line, an impedance bridge, rneans for connecting said bridge across the cord circuit after said sta-tion hangs up if the call is incoming over said trunk circuit, and means for preventingthe bridge from being placed across said cord circuit when the station hangs up if the call is outgoing.

6. In a telephone exchange system, a trunk line, a subscribers line and a cord circuit, each having a pair of talking conductors, a subscribers'station on said subscribers line, a talking circuit including the talking conductors of each of said circuits and said subscribers station, anY impedance bridge forv said cord circuit, and means. for rendering said bridge effective or not when the subscriber opens the line circuit after completion of conversation, depending upon Whether or not the call is incoming or outgoing from the subscribers station.

7 In a telephone exchange system, a trunk line, a subscribers line, a station thereon, a cord circuit for completing incoming calls over said trunk line for said station and for completing outgoing calls from said station over said trunk line, an impedance bridge for said cord circuit, means for connecting said bridge across said cord circuit, after conversation is completed if the call is incoming over said trunk line and means for, preventing the connection of said bridge across said cord circuit after conversation is completed if the call is outgoing over said trunk line.

8. In a telephone exchange system, a trunk line, a subscribers line, a station thereon, a cord circuit for completing incoming calls to said station over said trunk line and for completing outgoing calls from said station over said trunk line, a key in said cord circuit for signaling said station if the call is incoming over said trunk line, a supervisory signal in said cord circuit, a relay in said cord circuit contro-lled by the station for actuating said supervisory signal lto inform the operator that the station signalled desires to have the connection transferred, an impedance bridge for said cord circuit, and a second relay in said cord circuit controlled by said first named relay for connecting said bridge across said cord circuit.

9. In a telephone exchange system, a private branch exchange, a plurality of extension lines extending therefrom, a trunk line extending from said exchange to a main central office, a cord circuit at the private branch exchange for completing connections between an extension line and said trunk line or for interconnecting extension lines, a bridge for said cord circuit, means whereby, When said cord circuit is used in establishing a connection between an extension line and said trunk line, and a subscriber hangs up, the talking conductors of the cord circuit Will be opened and said bridge will be connected across the cord circuit if the call is incoming over said trunk line, and means whereby the talking conductors of the cord circuit Will be opened and said bridge Will be prevented from being connected across said cord circuit if the call is outgoing over` said trunk line.

l0. The method wherein calls are extended over a trunk and a cord circuit Which comprises furnishing different classes of supervision depending on the direction in which the call is extended.

In Witness whereof, I hereunto subscribe my naine this 17th day of March, A. D., 1921.

VALTER B. STRICKLER. 

